Protein A is a cell wall protein made by Staphylococcus aureus. The usefulness of Protein A stems from its ability to bind the Fc region of most mammalian immunoglobulin G (IgG) and, to a lesser extent, to immunoglobulin M (IgM) having a VH3 region, without affecting immunoglobulin affinity for antigen. Protein A is used commercially to purify monoclonal antibodies that are often in turn used as therapeutics in human diseases like inflammatory diseases and cancer. Also, Protein A can itself be used as a therapeutic. Protein A can be administered to a patient in order to bind circulating immune complexes in autoimmune diseases like rheumatoid arthritis or to stimulate specific cytokine production in someone with an infection. In addition, when coupled to a chromatography resin, Protein A can act as a therapeutic absorber to treat plasma or whole blood by removing IgG complexes in disorders like autoimmune disease and transplant organ rejection.
Originally, Protein A was derived from the S. aureus cell wall, but bacterial strains which secrete the protein have been isolated and are also used to obtain Protein A. However, the preparation of Protein A by conventional methods presents a problem for its use in human therapeutics. S. aureus is grown in media that contains animal products as a source of amino acids for the bacteria. In particular, bovine hydrolysates and extracts are commonly used as supplements in bacterial growth media. Hence, obtaining Protein A from growth media containing animal-derived products leads to the possibility that animal products like prions, the causitive agent in Mad Cow, Scrapie and wasting disease, could be present in the media and transmitted to human patients. Protein A preparations can also be exposed to and contaminated with animal products during downstream purification processes. For example, in methods currently used, Protein A is purified using an IgG Sepharose chromatography column and the IgG coupled to the Sepharose chromatography column is typically of animal origin. However, the animal products that come to be associated with a Protein A preparation during the isolation and purification processes are not easily purified from the preparation by the methods presently known.